They define on which weekdays your kitchen accepts Real Time orders. A day that is switched off:

• is not offered to customers on the /realtime page (the system looks for the first delivery date whose weekday is enabled);
• hides its Times block on the settings page;
• is not shown on the Real Time Menu calendar;
• is skipped by the nightly job that can auto-copy a previous Real Time menu.

These are the same weekdays shown under Realtime fulfilment ("Weekdays this kitchen fulfils realtime / on-demand orders") when you edit the kitchen in Kitchen Locations — saving either page updates the other. Pickup points additionally have their own Realtime Pickup Availability toggles, which can only be enabled for days the kitchen has on.